Transistor thermistor telemetering device



April 17, 1962 R. w. BURHANS ETAL 3,029,642

TRANSISTOR THERMISTOR TELEMETERING DEVICE Filed Dec. 22, 1954 I AumoAMPLIFIER 3,029,642 Patented Apr. i7, i962 ice 3,029,642 TRANSISTORTHERMISTOR TELEMETERING DEVICE Ralph W. Burhans, East Cleveland, andWarren Jackson, Jr., Lyndhurst, (Ehio, assignors to The Standard OilCompany, Cleveland, Ohio, a corporation of Ohio Filed Dec. 22, 1954,Ser. No. 477,076

13 Claims. (Cl. 73--362) This invention relates'generally totelemetering apparatus and more particularly to apparatus of thischaracter employing transistors. l

11n the prior art, one common form of telemetering apparatus employs asystem of relays, bells and buzzers to generate a signal indicative ofan undesired change in a physical condition monitored by the apparatus.Apparatus of this sort, however, requires a large number of parts, iscomplex in organization, is bulky, and has a maintenance schedule-requiring undesirably frequent visits. Other types of prior arttelemetering apparatus performing the same function utilize electroniccircuits containing vacuum tubes. The last-named type of apparatus isalso disadvantageous for monitoring a condition at a remote location,since the vacuum tubes cause a heavy current drain on the battery sourceof power usually used to energize the apparatus. Moreover, this Vcurrentdrain is aggravated by the characteristically long warm-up time requiredfor vacuum tubes to become fully operative. Hence, frequent `replacementof the battery is necessary. Note also that, since allowance must bemade for a warm-up period before each transmission of a data signal, thewarm-up period not only represents a current drain on the battery duringan interval in which the apparatus is performing no useful function, butin addition unduly extends the time necessary for a transmission. Y

=It is an object of this invention to provide telemeterng apparatus freeof the above-noted disadvantages of the. prior art.

A further object of this invention is to provide telemetering apparatuscharacterized by one or more of the advantages of'simplicity,compactness, ineXpensiveness, reliability, minimization of batterycurrent drain, and elimination of the necessity for a warm-up period.

Another object of the invention is to provide telemetering apparatus ofthe above-noted character suitable for monitoring a plurality ofphysical conditions at a remote location.

Yet another object of the invention is to provide telemetering apparatusof the above-noted character adapted to transmit signals telephonically.

These and other objects are realized in accordance with the invention byproviding an apparatus to be used at an unattended station and takingthe form of a transistor oscillator circuit and a means for sensing achange in the condition to be monitored. The transistor oscillatorcircuit is of a nature to provide for transmission of oscillatorysignals to an attended station at a frequency determined by an impedancecharacteristic of the circuit. The value of this impedancecharacteristic is changed by the sensing means in response to a changein the monitored condition. Accordingly, the oscillatory As anotherfeature in accordance with the invention, there may be providedcouplings of electrical or other nature for the telemetering apparatussuch that the same, in response to a telephonie signal received at theunattended station from the attended station, is adapted to establish atelephonie transmission, back to the attended station, of the datasignal for the monitored condition.

The invention may be better understood from the following detaileddescription of a representative embodiment thereof taken in conjunctionwith the accompanying drawing of the embodiment and wherein the electriccircuits and other components of the embodiment are shown partly inschematic diagram and partly in block diagram. While the invention isdescribed herein in connection with an application thereof formonitoring conditions in a mass spectrometer, it will be understood thatthe invention is of general application in the monitoring of one ormorephysical conditions.

Referring to the drawing, the numeral 10 designates a commerciallyavailable device known as an unattended station circuit or morepopularly as a ding-dong circuit. The ding-dong unit 10 (which is at theunattended station) has an assigned telephone number. The unit 10 isconnected through the telephone distribution network 11 with an attendedstation 12 when a subscriber at this latter station requests thenumberof the unattended station.

The ding-dong unit itself is not a part of the present invention. Hence,only a brief description of its more important components will be given.These consist of a microphone 13, a bell 14', a momentary start circuitshown as a movable contact 15 closable with a Xed contact 16,a-continuous stait circuit shown as a movable contact 17 closable with aiixed contact 18, and a stop circuit 19.

When a subscriber at attended station 12 wishes to obtain informationfrom the unattended station, he calls in the regular way the numberassigned to the ding-dong unit 10. The incoming telephonie signalreceived by the ding-dong unit is in the form of ringing current whichactivates the unit so that one short ringing interval is heard.Following the ring, the ding-dong unit operates the bell 14, whose toneis fed by air path 25 into microphone 113. TheV resulting ding-dong belltone is heard at the attended station for about a five-second interval.At the end of thisv interval, the bell stops, and momentary andcontinuous start circuits are actuated to close contacts 15, 16 andcontacts 17, 18.

The contacts of themomentary start circuit open after a short delay, butthe contacts of the continuous star-t circuit remain closed so long asthe ding-dong unit l0 is telephonically connected with the attendedstation 12. If the subscriber at station 12 hangs up, the contacts ofthe continuous start circuit are opened shortly thereafter to shut downthe ding-dong unit 10 as a whole. Alternatively, the unit may be shutdown by a signal originating at the unattended station in the form of acoupling together of a pair of external leads 26, 27 for the stopcircuit 19. When so shut down, the ding-dong unit l0 is then ready torespond to another incoming call.

While the ding-dong unit 10 is 0n, the microphone 13 thereof will pickup sonic signals for telephonic transmission thereof back to theattended station `12.. Accordingly, the ding-dong unit is adapted toprovide a telephonie connection with the attended station 12 of a sonicsignal generated by a telemetering apparatus.

Considering` the relation between the ding-dong unit ltr and the presentinvention, the momentary actuation to closure of contacts 1S, 16completes a circuit to energize a relay Winding 30 from a batterysourcer 31,.

Winding 30, when energized, closes a movable contact 32 with a fixedcontact 33 to complete a Circuit through the following elements: contact32, lead 34, junction 35, the parallel connection of a timing lmotor 36and a clutch coil 37 in a timing means y38, junction 39, lead 40,alternating current power source 41, lead 42, and the xed contact 33.When the mentioned circuit is so completed, the local power source 41(which may be a 110 volt A.C. source) will energize both the timingmotor 36 and the clutch coil 37.

The timing means 38 may be a` timer of a type described in bulletin 130Cpublished in March 1951 by the Eagle Signal Corporation of Moline,Illinois. In this timer, the clutch coil 37, whenA energized, links aplurality of normally open switches 51-56 to the rotation of timingmotor 36. Responsive to this rotation, each of the switches will closeduring the timing cycle for a fixed period thereof. The timer is suchthat it is possible to select, at will, the time of initiation and theduration of the closure period for any particular switch. lf, at anytime, the clutch coil 37 becomes de-energized, the linkage between theswitches and timing motor 36 is broken. Any closed switch then springsopen to thus restore the whole timerto starting condition.

The closure period for switch 51 is selected to last from the beginningto the end of the timing cycle. Hence, as soon as coil 37 is energized,the movable contact 51a of switch 51 closes with fixed contact 51hthereof to complete a self-holding circuit for the timer through thefollowing elements: movable contact 51a, junction 35, coil 37 and motor36 in parallel, junction 39, lead 40, A.C. power source 41, ,lead 60andfixed contact SIb. With this self-holding circuit, motor 36 ,andA coil37 stay energized despite opening of contacts 32 33 at the end ofmomentaryV actuation of contacts 157, 16.

The switch 51 at the end of a timing cycle will open to interrupt theholding circuit.v Uponoccurrence of this event, any closed ones ofswitches 51-56 will open. Thus, timing means 38 is self-resetting at thelend of a cycle. Moreover, upon interruption of the holding circuit, themotor 36 and coil 37 will be de-energized and cannot be re-energizeduntil relay winding 30 is again actuated by ding-dong unit 10. Thus, thetiming means 38 is also characterized by a single cycle feature in thesense that it is incapable of recycling to give more than one timingcycle for any given telephone call made to the ding-dong unit 10.

Within the ding-dong unit, the closure of the contacts 15, 16 of themomentarystartcircuitis accompanied by closure of the contacts 17, '18of the continuous start circuit. These Vlast-named contactsl remain'closed until the ding-dong unit is shut down by operationV of the stopcircuit 19. Shortly after closure of contacts 17, 18, the timing motor36 closes together the movable contact 55a and fixed contact 55b of theswitch 55, assigned a closure period lasting from the beginning to theend of the timer cycle. With this closure of switch 55, a circuit iscornpleted through the following elements: fixed contact 55h, lead 65,contact 18, contact 17, lead 66, v,battery 31, another battery 67, lead68, junction 69, lead 70, transistor oscillator circuit 71, lead 72,junction 73, lead 74, and movable contact 55a of timing switch 55. Whenkthe `mentioned V,circuit is so completed, the battery sources 31 and 67furnish power to the transistor oscillator circuit 71.

Considering oscillator circuit 71, while the same may be any one ofvarious transistor oscillators, preferably it is of the form to bedescribed. In oscillator 71, a transistor Si) having emitter, collector,and base electrodes 81, 82, 83 is connected so that battery currentflows through transistor 80 from emitter electrode 81 to collectorelectrode SZ. Inductance means, in the form of the center tapped,primary winding 79 of an audio-transformer 84, is coupled at one endwith collector electrode 82 so that vthe Vcenter tap 35 of the windingis coupled `with lead 72. Thus, the upper half of primary winding 79 i'scoupled with the emitter electrode 81 and the collector electrode 82 inthe path for the battery current.

To provide the A.C. signal feedback factor necessary for oscillation,the lower half of primary winding 79 is coupled to the base electrode S3by capacitance means in the form of a capacitor 86 having, say, a 0.05mfd. value. To complete the conditions necessary for oscillation, thebase electrode 83 is coupled with a resistance path permitting afraction of the current traversing emitter electrode 81 vto returnthrough the base `electrode 83 to the negative end of battery 31. Theserially connected, fixed resistor 87 (of, say, 10K value),a nd variableresistor 88 (variable, say, from O-ZSOK) form components of thisresistance path to be further described in greater detail. With thedescribed connections, the oscillator circuit 71, when energized,produces oscillatory signals at a frequency determined by 'an impedancecharacteristic of the circuit. This characteristic is, say, the RC valueof the resistance path yandthe capacitor 86. By changing the value ofthis impedance characteristic the frequency of the oscillatory signal iscorrespondingly changed. Preferably, the change in frequency is effectedby changing the resistance value of the resistance path. Also,preferably, the circuit parameters are selectedto give signalfrequencies within a particular range. This range includes frequenciesof audible value in the sense that sonic signals at these frequenciescan be heard by the human ear.

The oscillatory signals `are changed in frequency by sensing means whichmay take the form of a single sensing unit for monitoring asinglephysical condition. The sensing means in the presently describedembodiment, includes three sensing units. Of these sensing units, twoare in the form of the temperature sensitive resistors 90, 91 formonitoring the presence of liquid nitrogen in cold Vtraps (not shown) ofthe mentioned mass spectrometer. The third is in the form of a relay 92for monitoring the vacuum gauge 93 of the mass spectrometer.

The several sensing units 90, 91, 92 are coupled in turn into theresistance path of the transistor oscillator 71 by "the closurel throughtimer motor 36 of the switches 52,

53, 54. Taking up these lswitches in the order in which they close, whenswitch 52 closes, it completes the resistance path from base electrode83 to battery 31 through the following elements: fixed resistor 87,variable resistor 88, lead 95, switch 52, lead 96, temperature sensitiveresistor 90, lead 97, junction 98, lead 99, junction 73, and fromthenceback to battery 31 by the route given for the power circuit ofoscillator 71. The effect of closure of switch 52 is thus to route theresistance path for oscillator circuit 71 through temperature sensitiveresistor 90. As a first result the circuit 71 will start to oscillate.As a second result, the frequency of oscillation of the circuit isdetermined by the 'effective resistance of resistor 90. l

Temperature sensitive resistance 90 is of such type (well known in theart) that, when cooled to low temperature in the presence of liquidnitrogen, the resistor 96 has a `value of several megohms. With thismegohm value in the resistance fpath, an oscillation is produced at afrequency which in audible terms corresponds to a very low tone as, say,20-60 cps. This low frequency oscillation indicates the continuingpresence, as is desired, of liquid .s nitrogen in the cold trapmonitored by the resistor. On

the other hand, if no liquid nitrogen is present, the value of resistordecreases to several thousand ohms with the result that circuit 71oscillates at a relatively high frequency in audible terms as, say, at1000l cps. This high frequency value for the oscillatory signalindicates a failure in the action of the monitored cold trap.

lUnder the action of timing motor 36, the closure period of switch 52 iscompleted by the opening of the switch to decouple resistor 90 fromoscillator circuit 71. A short time thereafter', the motor36 closesswitch 53 for a period during which the resistance path for returncurrent from base electrode 83 to battery 31 is completed through thefollowing elements; fixed resistor S7, variable resistor 88, lead 95,switch 53, lead 105, temperature sensitive resistor 91, lead 97, andthen back to battery 31 in the same manner as previously described forresistor 90. Resistor 91, when so coupled with oscillator 71, controlsthe frequency of the signal thereof in a manner alike to the controllingaction of resistor 90. Thus, circuit 71 produces low and high audiblefrequency signals in respective correspondence with the presence andabsence of liquid nitrogen in theV cold trap monitored by resistor Thetiming motor 36 at the end of the closure period of switch 53 opens thisswitch to decouple resistor 91 from oscillator circuit 71. A short timethereafter, motor 36 closes switch 54 to complete the mentionedresistance path for circuit 71 through the heretofore-mentioned elementsup to lead 95', thence through switch 54, lead 106, reiay 92, lead 108,junction 98, and from thence back to battery 31 by the route heretoforedescribed. The closure of switch thus inserts the relay 92 into theresistance path for the third monitoring period.

Relay 92 includes a winding 110 connected across the Warning light 111for the vacuum gauge 93 of the mass spectrometer. The relay 92 alsocomprises a pair of r.fixed contacts 113, 114, the resistors 115 (of,say, 2M value) and 116 (of, say, 2K value) connected, commonly, with thelead '108 and, respectively, with fixed contacts 113, 114. The relay iscompleted by a movable ycontact 117 adapted to close with either of thexed contacts 113, 114.

When the mass spectrometer vacuum system is operating properly, therelay winding 1,10 will be energized by the voltage across warning light111. Winding 110, when so energized, maintains movable contact 117 inclosure with iixed contact 113 to maintain the niegohm value resistor1115 in the resistance path for oscillator circuit 71. Under theseconditions, the circuit 71 oscillates to produce the signal of normallow audible frequency. If, however, the mentioned vacuum system breaksdown, the warning light 110 goes out, the winding 110 becomesdeenergized, and the movable contact 117 opens with fixed contact 113and closes with fixed contact 114. By this shift in the movable contact,the low kilohm value resistor 116 is substituted for the megohm valueresistor 115 in the resistance path. Hence, in case of vacuum breakdown,the oscillator circuit 71 produces its warning signal of high audiblefrequency.

For conversion of the oscillator signal into a form convenientlytransmissible through the ding-dong unit 10, the secondary winding 1211of audio transformer 84 is coupled with the input lof an electrosonictransducer means 121. This transducer means may take the form of anaudio amplifier 122 operated by transistors, and a speaker 123electrically coupled with the amplifier 122 and sonically coupled by theair path 124 with the microphone 13 of ding-dong unit 1d. Conveniently,the amplifier may be served by leads 125,126 with power from the samecircuit which energizes transistor oscillator 71. Hence, the transistoramplifier will be energized during, and only during, the time intervalin which transistor oscillator 71 is energized.

During the respective periods in which switches52, 53 and 54 are closed,the continuous start contacts 17, 18 in ding-dong unit are kept closedto mai-ntain oscillator 71 lenergized through the power circuitheretofore described. Shortly afterA timing motor 36 opens switch 54 todecouple relay 92 from the oscillator, the motor 36 closes the switchV5'6 tothereby shunt together the exterior leads 25, 27 for the stopcircuit 19. Stop circuit 19 vin response to this shunting actionproduces the effects of telephonically disconecting the ding-dong unitfrom attended station 12, and of opening the continuous start contacts17, 18.

The opening of these contacts interrupts the power circuit to oscillator71 to produce de-energization thereof. Almost immediately thereafter,the timing motor 36, which is still running, causes -both the switches55 and 51 to open, to thus mark the end of the timing cycle. The openingof switch 51 interrupts the circuit from source 41 through motor 36 andclutch coil 37. The described telemetering apparatus is thus restored toits starting condition.

Considering some of the advantages of the described telemeteringapparatus, by limiting the energizatio-n of oscillator 71 and amplifier122 to the minimum necessary time duration, the current drain on thebattery sources is kept down, even though the subscriber calling theunattended station does not hang up his telephone promptly after thedata sign-al from the apparatus has been received by him. Conversely, ifthe subscriber' merely wishes to monitor the condition detected by thefirst sensing unit, the subscriber, by hanging up at the end of the datasignal transmission period for this rst sensing unit, can immediatelydeenergize the oscillator 71 through the opening of the continuous startcontacts 17, 18. In this manner, a further conservation of the currentdrain on the batteries can be eected. Note also that the timing means bytelephonically disconnecting ding-dong unit 10 (through the closure ofswitch 56), provides a convenient telephonie signal indicating to thesubscriber that full monitoring of the conditions of interest has beencompleted.

By the use of a transistor in oscillator 71 and of transistors inyampliier 122, the warm-up period required for the telemeteringapparatus is reduced substantially to zero. There is consequentcon-servation of battery current drain and of the time required fortransmission of the data signal. 'Ihe oscillator 71 itself, because ofits simplicity of circuit organization, provides a compact, inexpensiveand reliable means for obtaining data signals of physical conditions.This reliability of the oscillator, when taken together with the lowcurrent drain on the Ibatteries permits a maintenance schedule for theap-V paratus in which visits need be made no oftener than every sixmonths. Such visits will be primarily for the purpose of replacing thebatteries rather than for reconditioning the equipment.

In the event of power failure of the A.C. source 41, occurring beforeand lasting during a call made to the unattended station, the timingmotor 36 will remm'n unenergized with the result that switch 55 nevercloses to energize the oscillator 71 from the batteries. Accordingly,the subscriber will hear no audible tone, whatever, and will know fromthis fact of the power failure of source 41. Thus, the describedtelemetering apparatus provides for an indication of failure of localpower at the unattended station. In the event that the timing means 38is spring driven or run from the same batteries which energize theoscillator circuit, the same indication of local power failure can beprovided by a relay whose contacts are in series with switch 55, thementioned contacts opening upon the occurrence of a power failure sensedby the relay.

In the event that a power failure occurs in power source 41 duringtransmission of a data signal, the timing motor 36 stops and the clutchcoil 37 lbecomes deenergized with the consequence that all of switches51-56 return to open position. The timing means 38 is thereby restoredtostarting condition, and for the reasons given heretofore, cannot berestarted except by another actuation of the ding-dong unit 10 from`attended station 12.

to give data signals which are out of order and hence would bemisleading.

The above-described embodiment being illustrative only, it will beunderstood that the present invention comprehends organizationsdiffering in form or detail from the presently described embodiment. Forexample, in an application which requires the monitoring of only asingle physical condition, the timing means may take the form of asimple relaxation oscillator type of time-delay relay which is energizedfrom batteries Vfor the transistor oscillator and amplifier. As before,the time-delay relay places a time limitation on the energization of thetransistor circuits. Also, the invention is of application in ways otherthan with a mass spectrometer. For example, apparatus in accordance withthe invention may be used to monitor temperature at a remote spot of apipeline.

Accordingly, the invention is not to be considered as limited save as isconsonant with the scope of the tollowing claims.

We claim:

l. Telemetering apparatus for monitoring at least one physical conditionata remote location comprising, a transistor having base, collector andemitter electrodes, inductance means connected with two of saidelectrodes said electrodes defining a path lfor flow therebetweenthrough said transistor of current induced by an electrical energysource, capacitance means connecting' said inductance means and saidthird electrode for A.C. signal feedback to the latter, a variableresistance path connected with said third electrode to provide for'return therethrough to said source of a `fraction of the currentflowing from said source through said first-named. path, the aforesaidelements forming an oscillator circuit means to'develop oscillatorysignals at an audible frequency determined by the resistance of saidVariable resistance path, sensing means responsive to a change in saidmonitored icondition by reacting thereto to change the resistance insaid variable resistance path, said resistance change resulting in afrequency change in said oscillatory means to said connection to therebyproduce an answer to signals, means terminating a telephone connectionIfrom said remote location and responsive to an incoming telephonicsignal received therefrom to render said oscillator circuit energizedwith current from said source to thereby enable said circuit to developsaid audible frequency oscillating signal, and means to apply saidYaudible 4frequency signal through said terminating means to saidconnection to thereby produce an answer .to said incoming signal in theform of an audible tone developed at said remote location.

Z. A telemetering circuit as in claim 4l wherein said sensing means isin the form of temperature sensitive resistor means.

3. Telemetering apparatus for'monitoring a plurality of physicalconditions at a remoteV location comprising, a transistor having ibase,collector and Vemitter electro-des, inductance means connectedy with twoof said electrodes said electrodes dening a path for iiow therebetweenthrough said transistor of current induced by Velectrical energy source,capacitance means connecting said inductance means and said thirdelectrode for A.C. signal `feedback to the latter, a resistance pathconnected with said third electrode to provide for return therethroughto said source of a fraction of the current iiowing from said sourcethrough said iirst-named path, the aforesaid elements forming anoscillator circuit means to develop oscillatory signals at an audiblefrequency determined by the amount of resistance in said resistancepath, a plurality of variable resistance units Vfor sensing,respectively, said plurality of conditions, each sensing unit beingresponsive Vto a change in the associated condition by a reactionthereto in the :form of a change in the effective resistance of theunit, said resistance change resulting in a frequency change of saidoscillating signals, self-resetting timing means for connecting in turneach of said sensing units into said yes said incoming signal in theformof an audible tone cie-- velo-pcd at said remote location.

4. A telemetering circuit as in claim 3 wherein said timing meansinitiates and discontinues energization of said oscillator circuit fromsaid source at respective `times representing the beginning and end of atime interval within which said sensing units are successively connectedin saidresistance path.

5. Telemetering apparatus for monitoring at least one physical conditionat an unattended telephone station and for transmitting data on saidcondition to -an attended telephone station, said apparatus comprising,a transistor oscillator circuit means to provide when energized anaudible frequency oscillatory signal to be transmitted to said attendedstation, said Signal having a frequency determined by an impedancecharacteristic of said circuit means, Ysensing means responsive toachange in said monitored condition by reacting thereto to change saidcircuit impedance characteristic, self-resetting :single cycle timingmeans to control energization of said transistor oscillator circuitmeans'so as to limit such energization to a time interval ofApredetermined duration following initiation of operation of said timing`means, means at said unattended station terminating a telephone line eX-tension from said attended station and responsive to a telephone signalincoming therefrom to so initiate operation of said timing means, andmeans to apply said audible frequency signal through said terminatingmeans to said extension to thereby produce an answer to said incomingsignal in the form of an audible tone developed at Vsaid attendedstation.

6. Apparatus asin claim 5 further characterized by transistor amplifiermeans connected with said oscillator circuit to 'amplifyY saidoscillatory signals, the energization of said transistor amplifier meansbeing controlled by said timing means to be limited to a time intervalof the same duration as the energization interval off said oscillatorcircuit.

7. Telemetering apparatus for monitoring at least one physical conditionat an unattended station telephonically connectible with an attendedstation, said apparatus cornprising, a transistor oscillator circuitmeans to provide oscillatory signals within the yaudible frequency rangeand at -a frequency in said range determined by an impedancecharacteristic of said circuit, sensing means responsive to a change insaid monitored condition by reacting thereto to chan/ge Vsaid circuitimpedance characteristic, said last-named change producing an audiblechange in the frequency of said oscillatory signals, self-resettingsingle cycle timing means initiated into operation by an incoming signalreceived by telephone connection at said unattended station ifrom saidattended station for controlling the energization of said transistoroscillator circuit so as to limit such energization to a time intervalof predetermined duration following said incoming signal, and meansproviding for transmission of said oscillatory signals from saidunattended station over said telephone connection to said attendedstation.V

8. Apparatus as in claim 7 'wherein said timing means initiatesdiscontinuanceof the telephone connection between said unattended andattended stations 'after a time interval of predetermined durationfollowing the reception of said incoming signal. Y

9. Telemetering apparatus for monitoring a plurality of physicalconditions at an unattended station telephonically connectible with anattended station, said apparatus comprising, a transistor oscillatorcircuit means to provide oscillatory signals within the audiblefrequency range and at a frequency in said range determined by animpedance characteristic of said circuit, a plurality of variableimpedance units for sensing, respectively, said plurality of conditions,each sensingy unit being responsive to a change in the associatedcondition by a reaction thereto in the form of a change in the effectiveimpedance of the unit, self-resetting single cycle timing meansinitiated into operation by an incoming signal received by a telephoneconnection at said unattended station from said attended station forconnecting in turn the impedance of each of said sensing units into saidtransistor oscillator circuit, each connection so made producing anaudible change in the frequency of said oscillatory signals in responseto an impedance change of the connected unit, and means providing fortransmission of said oscillatory signals from said unattended stationover said telephone connection to said attended station.

10. Apparatus as in claim 9 further characterized by oscillatory circuitcontrol means responsive to a power failure at said unattended stationto maintain said oscil- Y lator circuit in a condition indicating saidpower failure.

1l. Apparatus as in claim 9 wherein said timing means initiates anddiscontinues energization of said oscillator circuit from said source atrespective times representing the beginning and end of a time intervalwithin which said sensing units are successively connected into saidoscillator circuit.

12. Apparatus as in claim 11 further characterized by means renderedoperative in response to telephonie disconnection at said attendedstation prior to the end of said interval to deenergize, forthwith, saidtransistor oscillator circuit.

13. Telemetering apparatus for monitoring a plurality of physicalconditions at an unattended station, said apparatus being adapted foruse with an unattended station device telephonically connectible with anattended station and having a transmitting microphone, a telephonecircuit opening means, and a start circuit actuated momentarily inresponse to an incoming signal received by telephone connection from theattended station, said apparatus comprising, a transistor oscillatorcircuit means to provide oscillatory signals within the audiblefrequency range and at a `frequency in said range determined by aresistance characteristic of the circuit, a plurality of variableresistance units for sensing, respectively, said plurality ofconditions, each sensing unit being responsive to a change in theassociated condition by a reaction thereto in the form of a change inthe effective resistance of the sensing unit, a timing motor adapted tobe initially energized by momentary actuation of said start circuit,self-holding means for continuing the energization of said timing motorthroughout a timing cycle, iirst connecting means operated by saidtiming motor for energizing said transistor oscillator circuit over aninterval of said timing cycle, said circuit being otherwse de-energized,a plurality of second connecting means operated successively by saidtiming motor within said interval to connect in turn the resistance ofeach of said sensing units into said oscillator circuit, each connectionso made producing an audible change in the frequency of said oscillatorysignals in response to a change in resistance 0f the coupled unit,electrosonic transducer means electrically connected with saidoscillator circuit and adapted to be sonically coupled with themicrophone of said unattended station device for providing transmissionof said oscillatory signals through said microphone and said telephoneconnection to said attended station, and means actuated by said timingmotor following operation thereby of lthe last of said second connectionmeans for initiating operation of the telephone circuit opening means ofsaid unattended station device.

References Cited in the file of this patent UNITED STATES PATENTS2,287,786 Diamond June 30, 1942 2,326,200 Bristol Aug. 10, 19432,418,836 Hawes Apr. 15, 1947 2,509,215 Craig May 30, 1950 2,550,109Derr Apr. 24, 1951 2,643,172 Reiss June 23, 1953 2,745,012 Felker May 8,1956 2,777,065 Pritchard Jan. `8, 1957 2,791,693 Moore May 7, 19572,845,547 Althouse July 29, 1958 OTHER REFERENCES Electronics, August1949, pages -91 entitled Transistor Oscillator,

